It is standard practice to make paper by a process comprising flocculating a cellulosic suspension by the addition of polymeric retention aid, draining the flocculated suspension through a wire to form a wet sheet, and drying the sheet.
Some polymeric retention aids tend to produce rather coarse flocs in which event retention and drainage may be good but the rate of drying of the wet sheet may be less satisfactory. In particular, the "formation" of the sheet may be rather poor. "Formation" is an indication of the arrangement of fibres within the sheet. A very uniform arrangement is referred to as good formation (but is generally associated with slow drainage) while an uneven distribution of fibres in the sheet is generally regarded as poor formation, but it tends to be associated with rapid drainage.
As a generality, it is known that increasing the molecular weight of a polymeric retention aid which is added immediately prior to drainage will tend to increase the rate of drainage but will tend to damage formation.
Normal practice dictates that polymeric retention aids polymers should be as soluble as possible and so they are usually formed by polymerisation of water soluble monomer or monomer blend in the absence of any cross linking or branching agent.
It is difficult to obtain the optimum balance of retention, drainage, drying and formation by adding a single polymeric retention aid and it is now common practice to add two different materials in sequence.
For a few specialised applications the preferred method is a "soluble dual polymer" retention system in which two water soluble polymers are used in sequence. Generally one has a higher molecular weight than the other. In each instance, both polymers are usually as linear and as soluble as is reasonably possible to achieve. Generally the first component is a high charge density, low molecular weight, cationic coagulant such as polyamine or poly DADMAC and the second is a low charge density, high molecular weight, dissolved anionic flocculant.
Different from "soluble dual polymer" retention systems are "microparticulate" retention systems in which a high molecular weight, first, polymeric retention aid (generally cationic) is added to the suspension, the resultant flocs are mechanically degraded by shearing or otherwise agitating the suspension, and the flocculated suspension is then reflocculated by addition of a water-insoluble, microparticulate, material which is usually anionic and counterionic to the polymer. Processes in which the microparticulate material is bentonite are described in EP 235,893 and have been commercialised under the trade name Hydrocol. The polymeric retention aid is usually a fully dissolved linear polymer but it can contain insoluble polymer particles in accordance with EP-A-202,780 or the process and material can be as described in U.S. Pat. No. 5,393,381.
It is also known to use silica and various silica compounds in place of bentonite, and to apply the insoluble microparticulate material after a soluble dual polymer system (low molecular weight cationic followed by high molecular weight anionic).
In EP 235,893 various water-insoluble polymeric emulsions were shown as being unsuccessful replacements for bentonite as the water insoluble microparticulate material. In EP 499,448 bentonite and various water insoluble microparticulate materials were mentioned for aggregating the flocs but it was also mentioned that a water soluble anionic polymer could be used instead of bentonite (thus creating a soluble dual polymer system).
U.S. Pat. Nos. 5,167,766 and 5,274,055 discuss known microparticulate systems wherein the insoluble microparticulate material is bentonite or silica sol and propose, two different microparticulate systems in which the microparticulate material is in the form of polymeric microbeads in the cellulosic suspension. Two different systems are described.
In one system the microbeads have a size of less than 60 nm, they are non-cross linked, and they are water insoluble as a result of having been made from water insoluble monomers. For instance the linear, insoluble, beads are exemplified by polystyrene.
In the other system, the microbeads have a size of less than 750 nm (0.75 .mu.m) and are described as being cross linked microbeads.
It is stated that the retention efficiency of these cross linked microbeads is due to "small strands or tails that protrude from the main crosslinked polymer" (American Cyanamid U.S. Pat. No. 5,274,055 column 4 line 4). It is clear that the cross linked microbeads in U.S. Pat. Nos. 5,167,766 and 5,274,055 are, as is conventional with microparticulate processes, water insoluble material which have sufficient structural integrity that they remain as microbeads despite the protrusion of small strands or tails from the microbeads into the solution. These patents emphasise the importance of the particle size being less than about 750 nm and there is data demonstrating grossly inferior results when the particle size is above 1 .mu.m. Since the size will be irrelevant if the beads dissolve, this is a further indication that the microbeads must function as particulate, undissolved, materials.
It is stated that the amount of cross linking agent which is used to make the microbeads should be "above about 4 molar ppm" and must be "sufficient to assure a cross linked composition" (American Cyanamid U.S. Pat. No. 5,274,055 column 5 line 22 and column 6 line 59) and is preferably 20 to 4000 ppm.
It is well known that it is possible to obtain by appropriate choice of the polymerisation materials and conditions either a branched water soluble polymer or a cross linked polymer which contains or consists of water insoluble material. For instance water soluble branched polymers are described in American Cyanamid EP-A-374,458. Whether or not a water soluble branched polymer (as in EP-A-374,458) or a water insoluble cross linked polymer (as in U.S. Pat. No. 5,274,055) is obtained depends upon the polymerisation conditions, the materials, and the amounts. American Cyanamid U.S. Pat. No. 5,274,055 discusses both cationic cross linked microbeads and anionic cross linked microbeads. The amount of cross linking agent used in the cationic microbeads is 100 ppm (examples 37 and 38) but the amounts of cross linking agent used for making the anionic microbeads are, in the examples, always more than this, and in particular they are always in the range 349 ppm to 1381 ppm. The only suggestion in U.S. Pat. No. 5,274,055 that lower amounts of cross linking agent might be usable to provide the cross linked microbeads arises from the statement that the microbeads may be made as in U.S. Pat. No. 5,171,808. That again teaches the use of an amount which is sufficient to assure a cross linked composition and which is at least 4 ppm and the examples all show amounts of from 349 ppm to 10000 ppm except for one anionic example with 97 ppm and one cationic example with 100 ppm.
It is stated in U.S. Pat. No. 5,274,055 that the cross linked microbeads preferably have a solution viscosity of 1.1 to 2 mPa.s. This is a saline solution viscosity measured on a 0.1% by weight polymer solution in 1 M sodium chloride at 25.degree. C. using a Brookfield UL adaptor at 60 rpm, and the anionic examples have values of 1.06 to 1.37 mPa.s.
It should be noted that, for convenience, we are using the word "solution" to describe the aqueous composition obtained by mixing the polymer with water and allowing the mixture to reach substantial equilibrium even though the "solution" necessarily contains undissolved microbeads. Accordingly we use the word "solution" throughout this specification to indicate a substantially equilibrated aqueous composition irrespective of whether it is a true solution or an aqueous dispersion of undissolved microbeads or larger beads.
We are aware of the commercialisation of materials for a microparticulate paper making process in which the final stage involves the addition of cross linked water insoluble microbeads under the trade name Polyflex CS and which we believe may be intended to be within the scope of U.S. Pat. No. 5,167,766 and U.S. Pat. No. 5,274,055. Scanning electron microscope examination of equilibrated solutions of Polyflex CS clearly shows the presence of substantially spherical undissolved particles having a size below 750 nm. This provides further confirmation that these two patents are intended to provide a water insoluble, microparticulate, retention system in somewhat similar manner to those provided previously using insoluble bentonite or silica sol. Thus, the cross linking of the particles prevents the particles dissolving into true solution during equilibration with water and instead they retain a visible microparticulate form, which is in the form of solid particles when dried.
These processes, using crosslinked insoluble microbeads, can give adequate retention and formation especially when they are conducted on cellulosic suspensions containing significant amounts of alum. However it would be desirable to obtain improved retention while maintaining adequate formation, compared to these processes. Additionally it would be desirable to obtain an improved combination of retention and formation compared to soluble dual polymer processes.